In recent years, due to advancement of advanced driving assistant system (ADAS) technology, the presence of a hazardous object, such as a pedestrian and a motorcycle, which has to be avoided by the driver of a vehicle with attention can be automatically detected, and the presence can be also conveyed to the driver by a voice message. In this case, the driver is further alerted that attention be paid for a hazardous object when a message for calling attention is heard from a direction in which the hazardous object is present. For example, when a hazardous object is on the right side of the driver, warning should be heard near the right ear of the driver. FIG. 6 is an illustration showing the manner in which a message for effectively calling attention is conveyed to a driver 61 in a vehicle 50. As illustrated in FIG. 6, for example, when a motorcycle on the right side of the vehicle 50 is approaching the driver 61 driving at a driver seat 51 of the vehicle 50, a voice message of “a motorcycle is approaching from the right” should be generated near the right ear of the driver 61. For example, when the vehicle 50 is crossing the left lane marker, a voice message of “the vehicle is crossing the left lane marker” should be generated near the left ear of the driver 61. In order to generate a voice message near the right and left ears of the driver 61 in this manner, it is sufficient to process an audio signal as needed, which is to be inputted to each of a left channel (left-side) speaker device 88 and a right channel (right-side) speaker device 89 provided in the vehicle 50.
In recent years, self-driving technology has been developed, and it is said that many of drive operation works can be automatically performed. However, it is expected that implementation of complete self-driving driving takes time, and assisting the driving operation of a driver using voice guidance and warning sound is considered to be continued.
When such voice guidance and warning sound overlaps with the sound from an audio device, and is hard to hear, which interferes with the driving operation. Thus, for example, the method shown in Patent Literature 1 is proposed as a method of controlling a sound field by adjusting sound field characteristics and an output sound volume so that the voice guidance and warning sound can be heard rather than the sound from audio equipment.